1. Field of the Invention
The invention relates to fluidized catalytic cracking.
2. Description of Related Art
In the fluidized catalytic cracking (FCC) process, catalyst, having a particle size and color resembling table salt and pepper, circulates between a cracking reactor and a catalyst regenerator. In the reactor, hydrocarbon feed contacts a source of hot, regenerated catalyst. The hot catalyst vaporizes and cracks the feed at 425.degree. C.-600.degree. C., usually 460.degree. C.-560.degree. C. The cracking reaction deposits carbonaceous hydrocarbons or coke on the catalyst, thereby deactivating the catalyst. The cracked products are separated from the coked catalyst. The coked catalyst is stripped of volatiles, usually with steam, in a catalyst stripper and the stripped catalyst is then regenerated. The catalyst regenerator burns coke from the catalyst with oxygen containing gas, usually air. Decoking restores catalyst activity and simultaneously heats the catalyst to, e.g., 500.degree. C.-900.degree. C., usually 600.degree. C.-750.degree. C. This heated catalyst is recycled to the cracking reactor to crack more fresh feed. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and for conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
Catalytic cracking has undergone progressive development since the 1940s. The trend of development of the fluid catalytic cracking (FCC) process has been to all riser cracking and use of zeolite catalysts. A good overview of the importance of the FCC process, and its continuous advancement, is reported in Fluid Catalytic Cracking Report, Amos A. Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8, 1990 edition of the Oil & Gas Journal.
Modern catalytic cracking units use active zeolite catalyst to crack the heavy hydrocarbon feed to lighter, more valuable products. Instead of dense bed cracking, with a hydrocarbon residence time of 20-60 seconds, much less contact time is needed. The desired conversion of feed can now be achieved in much less time, and more selectively, in a dilute phase, riser reactor.
The product distribution from modern FCC units is good. The volume and octane number of the gasoline is satisfactory, and the light olefins produced are upgraded via sulfuric or HF alkylation to high quality alkylate.
Unfortunately, refiners are finding it more difficult to make gasoline of sufficient octane and meet new specifications in regard to oxygenates, aromatics and benzene in the fuel. Reduced limits on RVP (Reid Vapor Pressure) and gasoline endpoint reduce the amount of butanes that can be added, further exacerbating the problem. Cost and availability of alkylation capacity make olefin alkylation a less attractive way to make gasoline.
We wanted to squeeze more gasoline and distillate out of FCC processing and change the quality and quantity of the light ends made by the FCC process. We wanted more light olefins, C3 and C4 olefins, but did not want to further reduce the yields of FCC gasoline. We wanted the increased light olefins for use in etherification and alkylation units, but were reluctant to do so at the expense of gasoline yield.
We also had to work within the constraints of existing units, many of which are limited in throughput by regenerator constraints, blower capacity to burn off the coke make associated with the cracking reaction, or too high a temperature in the regenerator.
The trend in modern FCC units is to operating conditions which favor production of large amounts of gasoline having a high octane number and relatively large amounts of light olefins. Unfortunately, the units have been pushed so far in these directions that further improvement is very difficult to achieve. Usually improvements in one area cause problems in another.
Higher riser temperatures, and shorter contact times in the riser reactor increase olefin yields, but also increase production of coke, dry gas and butadiene and reduces gasoline yields. Increased coke make increases temperature in the regenerator and puts more demands on the regenerator air blower. The increased production of dry gas can overload the wet gas compressor and the upper trays of the FCC main column. Increase make of butadiene makes the C4 streams less valuable, and increases acid consumption in downstream alkylation units.
We thought that conventional FCC processing had been pushed to the limit in regard to increasing olefin production, without increasing coke make, dry gas production or reducing gasoline yields inordinately. We knew there was a need for more light olefins, but conventional approaches, primarily directed at maximizing conversion of heavy feeds, had reached their limit.
Using existing technology, anything we did to increase C3/C4 olefin yield caused offsetting penalties in increased coke make, dry gas make or a severe loss of gasoline yield. Some of the conventional approaches will be reviewed below. Those primarily directed at better conversion of heavy will be reviewed first, followed by a review of additives which increase gasoline octane and yields of light olefins.